During the past year, we continued to study a group of 13 monoclonal antibodies prepared by immunizing rats with mouse brain. Nine have been partially analyzed. Four react with multipotential stem cells (CFUs). Only one of the monoclonal antibodies that kill CFUs also reacts with prothymocytes. Another of theae reagents reacts with all B cells. Two other antibodies have been extensively characterized; these react with, and are cytotoxic for, prothymocytes (AE3.38 and BG2.17). Another of these monoclonal antibodies, AE3.d3, is mitogenic for mouse lymphocytes. B lymphocytes are the most susceptible to this proliferative stimulus, and they are induced to differentiate into plaque-forming cells. The properties of a series of Thy-1-negative, cloned T-cell lines which can home to the thymus have been studied. These lines appear to be normal prothymocytes. The cloned prothymocytes are being used to investigate the nature of the "homing" signal which results in the repopulation of the thymus after irradiation. Much of the work this past year has been characterizing the photoactive reagent diazidobenzoyl cystine, an aryl azide containing both a radioactive isotope label and a disulfide capable of interacting with proteins. This reagent has been attached to a variety of proteins, and groups of mice have been immunized with each conjugate. Monoclonal antibodies were produced and used to generate ascites in mice. The resulting antibody was reacted with the photoaffinity-labeled antigen. T cells from the mice immunized with this antigen were also placed in culture, and lines that seem to have reactivity with diABC are being maintained. (LB)